The present application relates generally to turbocharger systems. More particularly, the application relates to turbocharger systems in which the dynamic performance of the turbocharger needs to be augmented with additional energy during certain transient operating conditions. Turbocharger systems in accordance with the present disclosure are applicable to all air-throttled engines (e.g., spark-ignition engines burning any type of fuel such as gasoline, ethanol, methanol, CNG, LNG, or LPG, and non-spark-ignition engines such as homogeneous charge compression ignition (HCCI) engines) equipped with a throttling device in the intake air system for air flow control.
The performance of turbocharged engines with throttled air flow systems poses several challenges. One such challenge is a result of the low amount of exhaust energy when the engine operates in a part-throttle mode, which leads to the turbine being unable to rotate fast enough, resulting in restriction of exhaust outflow. This imposes an unacceptable back-pressure on the engine and leads to consequential performance penalties. Another challenge is the adverse impact on the dynamic performance of the engine. This phenomenon is due to “turbo lag”, which refers to the inability of the turbocharger to increase its speed quickly enough under certain conditions, when the engine so demands. One such condition is when the engine is operating at a part-throttle mode and the throttle is suddenly moved to a fully open condition. In part-throttle operation, the exhaust flow rate and exhaust bound energy are very small. Hence, the turbine of the turbocharger rotates relatively slowly and thus the compressor provides very slightly boosted air. When the throttle is suddenly opened to produce more power from the engine, the turbocharger is unable to speed up quickly enough to be able to provide the demanded air flow. This slow response leads to a slow increase in engine power, causing inconvenience to the user.